I have a router that can use Wifi in b,g,n or bgn mixed mode.
I'm wondering if there are differences in signal 'strength' (and Electromagnetic field) concerning these protocols. For example, if i choose the 'b' version: is signal strength less and maybe 'safer' for health ?

3 Answers
3

I decline to comment on the "health" part of your question due to the extremely controversial and often unscientific public debate about the human health impact of non-ionizing electromagnetic radiation. However, your question does pose several good technical questions that are unrelated to health, so I will attempt to answer those without reference to biology/physiology.

Signal strength, as a concept, is intuitively defined as the amount of electromagnetic energy which was originally transmitted by the base station that is received by the receiver, compared to the amount of background "noise" (radio spectrum transmissions picked up by the receiver which do not originate from the transmitter).

So if you have a strong transmitter pushing out a lot of signal, but there's a lot of noise in between the transmitter and receiver, you get bad "signal strength", because it's hard for the receiver to clearly pick up the transmitter's signal over all the noise. Think of this like trying to verbally communicate with somebody when you are talking a few feet away from a running jet engine.

On the other hand, if you have a weak transmitter and very little background noise (i.e. the external environment is fairly "clean" of unwanted transmissions), it might be very easy to communicate even with low transmission energy. This would be like having a whispering conversation with somebody in an empty library with the air conditioner off; you both can understand each other perfectly despite speaking very softly.

The different 802.11 protocols don't particularly say "you can transmit at higher transmission energies" (which seems to be your question), but the newer protocols do devise smarter ways to use more of the electromagnetic spectrum to transmit more data, regardless of what the transmission energy is.

My advice to you would be to not worry about the "B/G/N" dichotomy if you are worried about the amount of transmission power. Instead, your goal should be to reduce the outside noise entering your wireless environment, and to make the base station (access point) as close as possible to your wireless client devices. These steps will make the devices themselves automatically and voluntarily reduce their transmit power, because, like two people talking in a quiet library, they'll be able to hear each other perfectly well without a lot of background noise.

To do that, you will want to make sure that there is a clear line of sight between your transmitter and receiver, and that you live in a place where there isn't a lot of external "chatter" on the wireless 2.4 and 5 GHz bands, such as the EM emitted by microwave ovens, cordless phones, and other wireless networks not controlled by you.

Alternatively, you can set up physical barriers that reduce or eliminate external radio frequency transmissions from entering your premises. I will not speak about the health impact of doing so, but I can factually state that setting up such a device (such as a Faraday cage) can reduce or eliminate other WiFi/bluetooth/microwave/cordless phone chatter in your environment, and this may make your WiFi more reliable and faster, while simultaneously reducing transmission strength.

Although there is increased range and throughput moving from 802.11b -> g -> n, these increases are realized through more than just antenna transmit power. Many of these gains are realized through different techniques (i.e. addition of MIMO in wireless N, or changes in bit modulation schemes).

For example, 802.11g introduced a new modulation scheme (OFDM) to attain higher bandwidth while simultaneously allowing for a reduced transmit power. This means that choosing 802.11b over 802.11g will actually cause an increase in the router's transmission power, as 802.11g is more efficient:

Compared to
802.11b, 802.11a and 802.11g consume much less energy for a given workload [...]

Further complicating matters, different hardware vendors will use different transmit powers on different devices. This is especially noted when moving from consumer to corporate grade equipment, where the transmit power is drastically increased (while still within the regulated limits).

While you can also overcome the default transmit power of many devices using a modified firmware, EM radiation is strictly governed in most areas. Maximum power limits are usually set by government agencies, and any COTS device you buy will be well within the power limit.

Finally, note that there are currently no known adverse health effects from being exposed to low-power electromagnetic waves. From the World Health Organization's website on EM fields:

With more and more research data available, it has become increasingly
unlikely that exposure to electromagnetic fields constitutes a serious
health hazard, nevertheless, some uncertainty remains.

[...]

International guidelines and national safety standards for
electromagnetic fields are developed on the basis of the current
scientific knowledge to ensure that the fields humans encounter are
not harmful to health. To compensate uncertainties in knowledge (due,
for example, to experimental errors, extrapolation from animals to
humans, or statistical uncertainty), large safety factors are
incorporated into the exposure limits.

A quick google search brought this article up. The article is concerned with wifi signals in general, as opposed to the differences between b, g, and n, but I think the info is just as useful. His concluding paragraph is below...

I conclude that levels of exposure of citizens to RF fields from
wireless networks is far below international safety limits. Moreover,
in nearly all of the places that I surveyed, the Wi-Fi signals were
far below other RF signals that were present from other sources. Given
the low level of exposure to people from RF fields from wireless
networks in comparison to that from other sources of RF energy that
are ubiquitous in modern environment, any health concerns about
wireless networks would seem to be moot.